Liquid synthetic rheology modifier polymers typically fall within one of three categories: alkali-soluble or alkali-swellable emulsion (“ASE”) polymers, hydrophobically modified alkali-soluble or alkali-swellable emulsion (“HASE”) polymers, and hydrophobically modified ethoxylated urethane (“HEUR”) polymers.
HASE and ASE polymers are generally known, see, for example those described in, U.S. Pat. No. 3,035,004, U.S. Pat. No. 5,292,843, U.S. Pat. No. 6,897,253, U.S. Pat. No. 7,288,616, U.S. Pat. No. 7,378,479, and US Patent Publication No. 2006/0270563, and have each been widely used as rheology modifiers in aqueous systems. However, some HASE polymers have shown deficiencies with respect to thickening efficiency, such as undesirably high sensitivity to relatively small variations in pH, electrolyte concentration, and the amount of polymer used. The thickening efficiency of such polymers in aqueous media tends to be low at low polymer concentration, for example, less than about 1% by weight polymer, particularly at low pH, such as for example, pH of less than about 6, but tends to markedly increase at higher polymer concentrations and/or higher pH. This sensitivity can lead to undesirably large changes in rheological properties, such as very dramatically increased viscosity, with relatively small changes in pH or polymer concentration. The disproportionately large changes in properties can lead to difficulty in designing a composition that has and maintains a desired performance profile under anticipated conditions of use, as well as to difficulties in manufacturing and handling such compositions. Cross-linked ASE polymers have also shown deficiencies with respect to thickening efficiency and thus may, particularly at low pH, require an undesirably large amount of polymer to provide the desired level of thickening, and, when used in an amount sufficient to provide the desired rheological properties, may impart a cloudy, translucent, or opaque visual appearance to aqueous compositions. A cloudy, translucent, or opaque visual appearance may be undesirable in end uses in which aesthetic criteria are important such as, for example, in personal care formulations, such as shampoos and body washes. Furthermore, some HASE and ASE polymers, such as some crosslinked alkali-swellable acrylate copolymers, typically exhibit a lower thickening efficiency and/or impart a cloudy, translucent or opaque visual appearance in the presence of salts and surfactants, which also limits the usefulness of such polymers in some systems, such as for example, personal care compositions.
There is an ongoing unresolved need for an associative polymer for use in modifying the rheological properties of liquid media, more typically aqueous media, that provides improved rheological, aesthetic, and/or application performance properties in such media.